We propose a combined developmental-comparative approach to the problem of retardation. Three populations--normal children, retarded individuals, and apes--will be compared whenever possible in perception, intentionality, representational capacity, language (both speech and reading), number, deductive reasoning, and transfer. Guided by what is known about the development of the normal child, we will ask the same two questions in each area: (1) Does the retarded individual follow the developmental stages of the normal child, but pass through them more slowly and stop earlier; or is he or she better described by a qualitatively different model? (2) How appropriate are animal models for the treatment of human problems? The three-way comparison should contribute to the development of a general model of intelligence, and to specific remedies for retardation.